1. Field of the Invention
The invention pertains to the field array antennas and more particularly to an array of coaxial elements arranged in a linear alignment.
2. Description of the Prior Art
A linear array of coaxial elements of the prior art comprises a plurality of coaxial cables, each having a solid dielectric between the inner and outer conductors, wherein the inner conductor of one cable is connected to the outer conductor of the succeeding cable. The wavelength of a wave propagating within a cable section is a function of the dielectric constant of the dielectric material and is given by .lambda./.sqroot..epsilon., where .lambda. is the free space wavelength and .epsilon. is the dielectric constant of dielectric material. Each section is one half a cable wavelength long. Since the dielectric on the outside of the cable is air, which has a dielectric constant that is less than that of the solid dielectric, the wavelength of a propagating wave in free space exceeds the cable wavelength. Consequently, each section is less than one half of a free space wavelength, the overall length being .lambda./2.sqroot..epsilon.. Performance of the prior art coaxial collinear array is degraded by the dielectric loading in three ways: first, the current distribution over the element sections is not uniform; second, the dielectric is lossy and contributes to antenna inefficiency; and third, the length per section is foreshortened, thereby adversely effecting the antenna gain. Further, the element sections are constructed of semi-rigid coaxial cable which must be cut to close tolerances, stripped at the ends, and the delicate operation of soldering the inner conductor of one section to the outer conductor of the next section performed. The soldering operation is especially difficult when the dielectric material has a low melting temperature such as polyethylene foam which is commonly used for its low loss characteristics.
A coaxial collinear array having an air dielectric between the inner and outer conductors of the coaxial elements is disclosed in U.S. Pat. No. 5,285,211 issued on Feb. 8, 1994 to Herper, et al and assigned to the assignee of the present invention. This array utilizes a coupler positioned between each half-wavelength section which has supports for the outer conductor and through passages for the inner conductors of adjacent sections. The outer conductor supports and the inner conductor through passages maintain the positioning of the inner and outer conductors without additional dielectric support, thus providing coaxial line sections with air dielectric. This design, however, still requires soldering of the inner conductor of one coaxial section to the outer conductor of its adjacent coaxial section. Though many of the soldering problems of the dielectric supported elements are solved, it is possible to have a cold solder joint which may be weakened by vibrations or stresses in shipment.